34 Lang, Stele of the Shoot of Isoetes lacustris. 



importance than the attainment of a clear conception of 

 how the growth at the apex gives rise to the resulting 

 mature structure. This is not made clear in the existing 

 descriptions and from some of the growing points figured 

 it is difficult to infer any way in which growth could take 

 place. The figures of Bruchmann, however, appear to be 

 wonderfully accurate, and though the apical region is 

 sometimes flat, I have usually found it forming a small 

 cone as he describes. His figures also show the arrange- 

 ment of the cells immediately below the growing point, 

 and allow of inferences being made as to the differentiation 

 of the mature stele from the procambium. 



Related to this question of the differentiation of 

 tissues behind the apex, is the anatomical question as to 

 whether there is a cauline portion of the stele upon which 

 the leaf-traces are inserted or whether the stem-stele is to 

 be regarded as wholly composed of united leaf-traces. 

 The latter view was expressed by Hofmeister and has 

 been held by a number of later investigators, while 

 Hegelmaier, Bruchmann and Scott and Hill regard the 

 stele as cauline. The important question is not merely 

 which term should be applied but what is the actual 

 relation borne by the leaf-traces to the stele. 



With this introductory survey of previous investiga- 

 tion, which will serve to bring out the points still more or 

 less in dispute, I may now describe the results of my 

 re-examination of the apex and stele of Isoetes lacustris. 



The study of numerous plants of Isoetes lacustris has 

 shown that while the actual apex of the stem is some- 

 times flat it usually forms a slight conical projection. 

 The appearance of this in longitudinal section is shown 

 in Photos i, 2, 3, 4 and 16. I do not propose to enter 

 here into the actual construction of the apical region 

 further than to say that while usually a small group of 



